Abstract
Identification of phosphoproteins or phosphopeptides as cancer biomarkers is an emerging field in phosphoproteomics. Owing to the low stoichiometric nature of protein phosphorylation, phosphoproteins or phosphopeptides must be enriched prior to downstream mass spectrometry analysis. Titanium dioxide (TiO2) has been prevalently used to enrich phosphopeptides from complex proteome samples due to its high affinity for phosphopeptides, and the method is straightforward. In this protocol, an offline phosphopeptide enrichment procedure using TiO2 columns is described. Peptides from a proteome lysate are loaded onto a TiO2 column in an acidic environment, followed by column washing with aqueous, organic, and ammonium glutamate (NH4Glu) buffers at acidic conditions. Phosphopeptides are eluted using an ammonia solution at high pH. Use of NH4Glu significantly reduces nonspecific bindings while a high recovery rate (84 %) of phosphopeptides is retained. The method is optimized for large-scale phosphoproteomic analysis and phosphoprotein biomarker discovery starting from sub-milligram or milligrams of proteome samples.